Jason Reed/Reuters

The full Moon is making everyone crazy. More people get arrested when the Moon is full.The new Moon is making everyone depressed.

Maybe you’ve heard things like that. Maybe you’ve said them yourself. It seems plausible that the second-brightest thing in the sky, the closest astronomical body to Earth, and the object largely responsible for the tides, could cause measurable changes in human behavior. After all, some animals coordinate their behaviors with the phase of the Moon.

As a result of this style of thinking, hospital workers will sometimes claim more births or injuries happen, police will notice more arrests, mental health professionals will feel their clients change behaviors, and so forth, depending on the Moon’s phase. Despite that, repeated studies have shown no strikingly different behavior: there aren’t big differences in car wreck frequency, births, murders, or depression incidents between the new Moon and full Moon.

The latest round of these studies started when researchers decided to investigate reports of higher admissions during the full Moon to a hospital in Spain. Specifically, they were looking at gastrointestinal bleeding, and they found a very small increase in the number of patients when the Moon was full, over other days during the month.

The study was fairly small in scope—just over two years of data for one hospital—and as the authors themselves pointed out, any possible explanation for internal bleeding involving the Moon seems pretty far-fetched. Nevertheless, any research showing a connection between the Moon and human health is a pretty interesting deal, so a University of California-Los Angeles astronomer named Jean-Luc Margot decided to check their numbers.

Margot performed another set of statistical tests and found the small bump in hospital admissions at the full Moon disappeared. (As a physicist by training, I think these two papers are the first I’ve ever read in nursing journals). He uses this as a springboard to point out how easy it is to be fooled by small apparent patterns hiding in what is probably random data.

Margot’s primary concern is very sound: we humans have a habit of looking for patterns. Mostly it’s a helpful habit: things that repeat in predictable ways help us survive, learn, and discover new things. Sometimes, though, we think we see patterns that just aren’t there. Margot argues that the expectation that things might be different during a full Moon led the researchers to find a very small effect in their statistical results.

While I’m pretty sure his conclusions are sound, Margot comes out sounding like a big jerk. Much of his paper is devoted to criticizing the methodology of the Spanish study in a condescending way—up to hinting that the researchers should have recruited an astronomer to help them do the statistics. I’m somewhat more inclined to be sympathetic, even though I don’t think there’s any reason to believe the Moon has any role to play in gastrointestinal bleeding.

Science is based on hunting for patterns and building predictive models to extract meaning from them. When the data is messy— it’s difficult to do statistics and hard to trust your results. The solution is to be as skeptical as possible, and that’s where I fault the Spanish researchers. Their study produced a marginal result based on a small population, and there’s good reason to think there should be no result at all. That’s two big reasons to doubt.

The phases are the result of the relative positions of the Moon to Earth. When the Moon is between us and the Sun, we see a new Moon: the dark side. When the Moon is on the opposite side of Earth from the Sun, we see it full sunlight, which is a full Moon. (The orbit of the Moon is tilted compared with Earth’s path around the Sun, so most of the time the Moon doesn’t block the Sun during a new Moon phase. On rare occasions it does, and that’s a solar eclipse.)

The only relevant difference between a new and full Moon, then, is the amount of light we see reflected off the Moon’s surface. That might be enough to influence the behavior of (say) night-hunting animals and their prey, but there’s little reason to think it might affect internal bleeding. Earlier statistical studies showed that more plausible ideas—episodes of depression and so forth—don’t correlate with the Moon, either.

It’s easy for someone to mock another person for blaming the Moon for something it has no effect over; I’ve done it myself, though I’m trying to do better. The important thing to remember is that we’re all human, we all look for patterns and are vulnerable to seeing ones that aren’t there…and try to help each other do better at distinguishing between what’s really there and what is just a trick of the Moon.